Shear layers of a circular cylinder with rotary oscillation

The behavior of the separated shear layers and the near wake of a circular cylinder with small-amplitude rotary oscillations (Omega(1) = 0.05-0.15 for f (f)/f (o) <= 1.25) were investigated experimentally at Re = 3,700. Measurements of an unforced cylinder were also made for 2,000 <= Re <= 10,000 to better understand the effects of rotary oscillations. The results show that the shear-layer vortices formed closer to the cylinder and the distance separating them was found to decrease with cylinder oscillations. The shear-layer frequency, however, increased with increasing forcing frequency f (f). The formation-region length l (f) decreased significantly with increasing f (f) while decreased to a lesser extent with increasing normalized oscillation amplitude Omega(1). The shear layer also diffused to a length L (d) larger than that of an unforced cylinder, while the l (f)-L (d)-Strouhal frequency offsetting mechanism was generally maintained. The near wake was of lower momentum compared to an unforced cylinder, and the transverse velocity fluctuations associated with the unforced vortex-shedding frequency f (o) always presented a local peak at f (f)/f (o) = 0.5, regardless of Omega(1) tested.